What is basal metabolism

Basal metabolism is the most basic energy needed to maintain normal vitality. This energy is generally obtained from the food the human body eats. Through its own normal metabolism, the nutrients in the food are converted into the energy needed by the human body to maintain normal life activities. Generally speaking, men have a higher metabolic rate than women. The metabolic rate of normal people is highest in their twenties, so the metabolic rate is also a representation of vitality.

Basal metabolism is the most basic energy consumption to maintain the body's vital activities. The energy expenditure of basal metabolism constitutes an important part of the body's energy expenditure and is an important basis for studying the energy expenditure and energy needs of the human body [1].

Basal metabolic rate for men

The basal metabolic rate varies physiologically with gender, age, etc. The basal metabolic rate of men is on average higher than that of women, and it is higher in children than in adults; the older the age, the lower the metabolic rate. Generally speaking, it is normal if the actual value of the basal metabolic rate differs from the normal average by 10-15%. It is considered a pathological state only when it exceeds the normal value by 20%. When the thyroid gland is hypofunctioning, the basal metabolic rate is 20-40% lower than the normal standard; when the thyroid gland is hyperfunctioning, the basal metabolic rate is 25-80% higher than the normal standard. When other functions such as adrenal cortex and pituitary gland are impaired, the basal metabolic rate will also decrease.

The minimum power required to maintain life

Abbreviated as BM, it is the minimum power required to maintain the life of the body. In order to eliminate the additional energy consumption (called functional consumption) caused by muscle work, mental activity, food digestion and temperature regulation of external temperature changes, the person being measured must remain absolutely quiet and fast (humans do not eat for 12-18 hours), and the energy consumed at the critical temperature is used as the basal metabolic rate. The basal metabolic rate of adults per day is 1200-1400 kcal for Japanese and 1500-2000 kcal for Europeans and Americans, but due to daily lifestyle and other influences, the calorie count is not absolute. The basal metabolic rate inevitably includes the mechanical work of the heart, respiratory muscles, digestive tract and vascular smooth muscles, as well as the functional consumption caused by the secretory activities of the liver and kidneys. The basal consumption caused by the life process of each cell accounts for about 3/4 of it. For the same warm-blooded animal in the same environment, its basal metabolic rate is proportional to its body surface area. A person's body surface area S (cm2) can be calculated from weight W (kg) and height H (cm) using various experimental formulas. Dubois' (1915) formula: S = W0.245 × H0.725 × 71.84 is widely used. For Japanese, the appropriate coefficient is 72.46 for females and 74.49 for males. Of course, the basal metabolic rate per unit body surface area also varies with age, gender, time of day, season, nutrition, physical condition, medication, pathological condition, etc. It is lowest when a person is born, then increases rapidly, reaches the highest value at the age of 5, and then gradually decreases, and remains roughly constant between the ages of 20 and 40 (as noted above). Then it slowly decreases again in old age. The value for adult women is 5-7% lower than that for men, which is because women have a higher proportion of fat tissue in their bodies. The difference between the measured value and the normal standard value (for each age and gender) as a percentage of the normal standard value (+: increased metabolism, -: decreased metabolism) is called the basal metabolic rate (BMR), which is often used in clinical practice for thyroid function tests, etc. [1].

Minimum energy metabolism

The basal state is when the human body is fasting, lying flat, awake and quiet at room temperature of 18 to 25 degrees Celsius. At this time, the minimum energy metabolism required to maintain basic life activities such as heartbeat and breathing is called basal metabolism (BM). Its value is related to gender, age, height, weight and health status. As mentioned before. All the energy produced by the body is eventually converted into heat energy. Therefore, in order to compare the levels of energy metabolism in different individuals, the amount of heat dissipated by the body per square meter of body surface area per hour (kJ/h·m2), i.e. the basal metabolic rate (BMR), can be used to express it.

The body surface area (S) can be calculated from the following formula:

S(m2)=0.0061×height(cm)+0.0128×weight(kg)-0.1529

Basic metabolic meter

Basic metabolic meters are commonly used in clinical practice to measure oxygen consumption per unit time (O2L/h). Because the conditions are relatively fixed in the basic state, through investigation, the basal metabolic rate (BMR) can be obtained by substituting the mixed respiratory quotient of 0.82 and the oxygen thermal value of 19.3kJ into the following formula.

BMR=O2L/h×19.3kJ÷S

For example, a 30-year-old male with a body surface area of ​​1.5 m2 has an oxygen consumption of 12 L/h under basic conditions.

BMR = 12L/h × 19.3kJ ÷ 1.5m2 = 154.4kJ/hm2

Then, according to the normal BMR values ​​for other age groups in Table 9-3, calculate the percentage of excess (+) or deficiency (-) to determine whether it is within the normal range (generally ±10%).

Basic heat production

The basal metabolic rate of normal people is relatively constant, generally slightly higher in men than in women, higher in children and young people than in adults, and gradually decreases after adulthood.

Common factors

Many factors can affect the heat production process, and only the most common ones are listed here.

Basic metabolism

For example, patients with hyperthyroidism have increased heat production and an increased basal metabolic rate, while patients with hypothyroidism have a decreased basal metabolic rate.

Muscle activity

When muscles are active, the heat generated by bones can increase several times and account for 75% to 80% of the total heat production. The extent of the increase is related to the intensity of muscle activity. For example, when walking, the energy level increases by about 3 times compared to a quiet state, while during strenuous exercise, it can increase by 10 to 20 times. When the body is engaged in busy mental work, it can strengthen skeletal muscle tension and adrenal activity through neural pathways, and also increase heat production.

The special dynamic effects of food

The body produces an additional amount of heat for a period of time after eating compared to before eating. Protein foods can increase heat production by an additional 30%, while carbohydrate or fat foods can increase it by 4% to 6%. When people eat ordinary mixed food, the heat production increases by 600 to 800 kJ per day. The mechanism by which this phenomenon occurs is not fully understood.

Ambient temperature

The human body's energy metabolism is most stable in an environment of 20 to 30 degrees Celsius. When the temperature is above or below this range, heat production increases. When the human body is stimulated by cold, it reflexively causes increased muscle tension first, followed by a chill reaction. Shivers are involuntary, small, rhythmic contractions of skeletal muscles. Its characteristic is that the extensor and flexor muscles are active at the same time, and almost no external work can be done. At this time, all the energy consumed is converted into heat. The maximum heat production rate can reach 39.2kJ/kg per minute, which increases the body's heat production by 4 to 5 times compared to usual. When the temperature is between 30 and 45°C, the body's heat production also increases, which may be due to the increase in the speed of chemical reactions in the body at this time [2].

Activity of endocrine glands

As mentioned above, thyroid hormone can enhance oxidative metabolism. Epinephrine can also enhance intracellular oxidative reactions, causing increased blood sugar concentrations and enhanced blood sugar utilization, thereby increasing heat production.